This invention relates to an active filter, and more particularly to an active filter used as a phase equalizer.
Active filters have been used for video tape recorders (VTRs) and television (TV) receivers. This type of filter may be used as a band emphasis filter (BEF), band deemphasis filter (BDF) or phase equalizer, depending on its gain characteristic.
The phase equalizer generally means a circuit whose gain characteristic is fixed and not dependent on the frequency, and phase only is variable.
The transfer function of this circuit is generally expressed by a complex number whose numerator and dominator are conjugate to each other as given below ##EQU1## where K is a real constant. The gain characteristic G(.omega.) and phase characteristic .phi.(.omega.) of this circuit are ##EQU2## As seen from the above expressions, the phase equalizer has the phase equalizing function.
In a practical use of this type of filter, input signal X(S) is supplied to the positive input terminal of a first operation amplifier (OP AMP) of the differential input/output type. The positive output terminal of the first OP AMP is connected to the positive input terminal of a second OP AMP of the differential input type. The positive input terminal of the second OP AMP is connected to one terminal of a first capacitor. The other terminal of the capacitor is grounded in an AC mode. The positive output terminal of the second OP AMP is connected to the negative output terminal of the first OP AMP, and connected to the positive input terminal of the first OP AMP, through a second capacitor. The positive output terminal of the second OP AMP is connected to the input terminal of a buffer. The output terminal of this buffer is connected to the negative input terminals of the first and second OP AMPs. The output terminal of the buffer provides output signal Y(S).
The input/output characteristic of the active filter is given by ##EQU3## where gm.sub.1 and gm.sub.2 are the incremental transfer conductances of the first and second OP AMPs, and C.sub.1 and C.sub.2 are capacitances of the first and second capacitor. In the equation (1), if the conductances gm.sub.1 and gm.sub.2 are selected as below, EQU gm.sub.2 -gm.sub.2 =gm.sub.1 ( 2)
the equation (1) describes the phase equalizer characteristics. In the equation (1), if S=j.omega., the equation (1) can be rewritten into ##EQU4## where .omega. is an angular frequency, and j indicates the conjugate.
The gain characteristic is defined as an absolute value of the transfer function, and therefore we have ##EQU5## Substituting the equation (2) into the above equation, we have ##EQU6## The above equation indicates an all-pass filter, which has the gain of 1 and is independent from the frequency. The phase characteristic of this filter is ##EQU7## Substituting the equation (2) into the above equation, we have ##EQU8## The above equation describes the characteristic of the phase equalizer whose gain is fixed and phase only is variable.
In realizing the active filter as mentioned above, we can easily consider a most primitive OP AMP made up of four transistors and a single constant current source. First and second transistors are paired. The first transistor is an NPN transistor, which is connected at the emitter to the constant current source, and connected at the base to the negative input terminal of the OP AMP. The second transistor is an NPN transistor, which is connected at the emitter to the constant current circuit, and at the base to the positive input terminal of the OP AMP. Third and fourth transistors are paired. The third transistor is a PNP transistor whose emitter is connected to an operation power supply and collector is connected to the collector of the first transistor. The fourth transistor is a PNP transistor whose emitter is connected to the operation power supply and collector is connected to the collector of the second transistor. The bases of the third and fourth transistors are connected together and to the collector of the fourth transistor. The output (positive) of the OP AMP is derived from the collector of the third transistor.
The basic OP AMP thus arranged has one terminal as the output terminal. To construct the OP AMP with positive and negative output terminals, as in the first OP AMP in the active filter, two basic OP AMPs are needed. Therefore, to construct the active filter by using the basic OP AMPs, three OP AMPs must be used. Thus, the filter circuit is made complicated. If the filter is fabricated into an IC circuit, it is advantageous if its circuit construction is simplified.
To use the active filter as the phase equalizer, viz., to obtain the phase equalizer characteristics, it is necessary to equal the difference between the incremental transfer conductance gm.sub.1, and the conductance gm.sub.2 to the conductance gm.sub.1.
To realize the equation (2), gm.sub.2 -gm.sub.1 =gm.sub.1, the ratio of gm.sub.1 and gm.sub.2 must be set to 1:2, exactly.
Since this factor gm is determined by all of the transistors and the constant current source in the OP AMP, to obtain the well paired conductances gm.sub.1 and gm.sub.2, the constants of all of the elements must be exactly equal between the first OP AMP and the second OP AMP.
The prior art circuit can not realize the equation (2) satisfactorily. Thus, it is a present technical problem as to how the active filter can be IC fabricated.